In the scene where Dodds (Warren Donnelly) enters the outside container, you can see "Del Boy" graffitied onto the wall. Clearly a BBC nod to Only Fools and Horses (1981) however this was subsequently set 8 years prior to the UK knowing who Del Boy existed.
Gene Hunt bets Sam Tyler "a tenner" (10 British pounds sterling) that he can solve a murder first. In 1973, 10 British pounds had the purchasing power of $72.80 in 2006, when the series first aired, so it's a quite substantial wager.
In the opening scene, Sam Tyler tells a kid to ride on the street or he'll "come round and stamp on your toys". This is the same phrase Gene Hunt uses in the opening episode that Tyler rolls his eyes at. This is a sign that Sam is slowly becoming more acclimated to his life in 1973.